Ferrite modulators



March 14, 1961 W, W, MCLEOD, JR 2,975,379

FERRITE MODULATORS Filed OC'C. 30. 1957 Trop/VFY United States Patent O FERRITE MoDULA'roRs Willard W. McLeod, Jr., Lexington, Mass., assignor to Raytheon Company, a corporation of Delaware Filed Oct. 30, 1957, Ser. No. 694,699

15 Claims. (Cl. 332-51) This invention relates to waveguide rotating devices, and, more particularly to waveguide rotating devices of the non-reciprocal ferrite type which combine the functions of load isolation, and of frequency and amplitude modulation of a microwave source in an electromagnetic wave transmission system.

Ferrite rotators utilizing the Faraday rotation effect have been used as circulators, in combination with a cross-polarized rectangular input and output T section, to rotate a plane or linearly polarized wave at microwave frequencies through a predetermined angle for isolation of the load from the microwave source. When a ferrite rotator of this type is used to introduce the desired rotation, in a square or circular waveguide, the propagated wave, upon emergence from the rotator, is polarized at a different angle from that of the original wave, and requires that the output section of waveguide to which the rotator is connected be rotated and oriented with respect to the input section of waveguide by an amount equal to the polarization rotation of the electric vector produced by the ferrite rotator. In this manner, energy entering the input port of the input T section of the waveguide `will be rotated, for example 45, in the ferrite section and leave by the output port of the output T section which is connected to the load, while energy rellected from said load back to the microwave source will be isolated from the microwave source and will appear in the cross-polarized arm of the T section adjacent to the microwave source. This reliected energy is absorbed by proper termination of the cross-polarized arm of this T section, as well as in the arm of the output T section, which is not connected to the load or source. A 45 load isolator of this type, which has proved useful in preventing oscillator pulling when operated into mismatched loads, is described in detail in an article by A. G. Fox et al. entitled, Behavior and Applications of Ferrites in the Microwave Region, in the Bell System Technical Journal of January 1955, pp. 38v39.

In numerous applications, however, it would be desirable to combine the function of isolation with the function of modulation of the microwave source by utilization of the Faraday rotation effect to provide an accurately controlled percentage of either frequency or amplitude modulation of said source.

In accordance with the invention, a ferrite circulator which combines the function of modulation of a microwave source and the function of load isolation can be achieved by providing a ferrite element mounted in a circular waveguide, or along the longitudinal axis in a square waveguide, and providing a magnetic field-producing coil in the region of the ferrite element, to which is applied a direct current for obtaining a lixed percentage of polarization rotation and an alternating current for obtaining a controlled deviation of the polarization of microwave energy or voltage above and below the fixed polarization rotation, and in directing this deviated voltage toward the microwave source to produce frequency modulation or ampltiude modulation according to the reactive or resistive character of the load presented to the microwave source. For directing this deviated voltage toward the microwave source, input and output T sections are connected to either side of the rotator section of waveguide, each T section having a pair of crosspolarized or polarization-selective arms, and being physically rotated with respect to a single polarization by the angular distance, preferably 45, determined by the direct current field, one arm of the first or input T section being connected to a source of microwave energy and the first arm of the second or output T section being connected to a load. The other arm of the first or input T section extends in a direction perpendicular to the axis of the rst arm and is terminated in a matched termination. The remaining arm of the second or output T section is provided with a slidable shorting element, which may be adjusted to an eighth or quarter multiple of the wavelength to present a resistive or reactive deviation or mismatch to the microwave source and, thus, to produce either amplitude modulation by changing the resistive loading and, hence, the effective coupling between the microwave source and the load, or, alternately, to produce frequency modulation by adding or removing a reactive load from the source in accordance with the polarization deviation of the microwave energy in the ferrite rotator. For example, a fixed rotation of 45, as determined by the direct current field in the ferrite rotator, is deviated by the alternating current field for an angular distance of one or two degrees, in order to pull the frequency of the microwave source according to the frequency of the applied alternating current field, the shorting element in the first section being adjusted to an even multiple of quarter wavelengths. ln this manner, controlled percentages of pure frequency modulation or amplitude modulation can be provided from a microwave source, which, when modulated by conventional plate modulation, usually produces a mixture of frequency modulation and amplitude modulation. Moreover, relatively extensive modulation and power supply equipment normally required to produce this modulation can be replaced by this relatively low-power waveguide rotation device.

Other objects and advantages will be more readily perceived upon analysis of the drawing, in which:

Fig. 1 is an isometric view, partly in section, of the ferrite modulator according to the invention;

Fig. 2 is a graph showing the frequency deviation of the microwave source as a function of the modulator impedance;

Fig. 3 is a graph showing the amplitude modulation of the microwave source as a function of the modulator impedance.

Referring now to Fig. l, a ferrite circulator is indicated generally by reference numeral 10, and includes an output T section 12, a ferrite or circular polarizer or rotator 14, and an input T section 16 of waveguide, the main arm of which is connected to a microwave source of energy 18. More particularly, Fig. 1 shofws a rectangular input section of waveguide 20, adapted to be connected to an antenna 22, or other desired load, supported by legs 23 attached to the dish portion of the antenna, and having a rectangular input waveguide section 24 adapted to be coupled to input section 20. For purposes of keeping the drawing to a reasonable size, the antenna is shown drawn to a different scale, the waveguide 24 being of a size sufficient to couple to input section 20. A ange 25 is shown, which couples the rectangular waveguide section 20 to the square portion 2,6 of the T section 12, and which functions as an input section of waveguide and includes a rectangular arm 27, which cooperates with the rectangular input section 20 to provide a pair of eonjugately-related terminals or branches in that a wave launched in either one will not appear at the other.

The rectangular arm 27 is provided with a flange 28, which may be integral therewith, and having holes 29 for coupling the arm 27 to other waveguide sections. Rectangular arm 27 is further provided with a tapped section 30, soldered to flange 28, and adapted to receive a threaded adjustable screw 31, to which is connected a slidable short-circuit bar or plunger 32, for producing a resistive or reactive mismatch, as will be presently described.

The ferrite rotator 14 is connected to the square portion of the T section of waveguide 26 by means of a waveguide flange 33 connected to rotator flange 34 by screws 35.

It should be understood that any standardtype rotator may be used, provided the proper polarization and mode of energy is thereby obtained. 'Moreoven it is obvious to one skilled in the art that a number of other wellknown rotation means may be employed.

In the present instance, rotator 14 includes a cylindrical ferrite element 36, positioned within a section of circular waveguide 38 by means of low-loss sections of dielectric material 39 and 39a, such as Teflon, which act as a solid supporting medium for the ferrite element 36. The Teflon dielectric material may be cut to the inner diameter of the circular waveguide section 38 divided into the two sections 39 and 39a, as shown at 40 in the region of the ferrite element, a hole bottom-drilled into each section, and the ferrite element slidably inserted into the Teflon. Many other methods for mounting the ferrite within the Teflon or other `materials will suggest themselves to those skilled in the art. For example, when the Teflon is to be inserted into a curved section of the waveguide, the Teflon may be heated until soft and poured into that portion of the waveguide.

The ferrite device 14 further includes a magnetic fieldproducing means, such as field coil 42, surrounding the circular waveguide 38 in the region of the ferrite element 36. The field coil 42 may consist of 10,000 turns of wire connected to separate power supply means for obtaining both static and dynamic polarization rotation from the ferrite rotator 14. As shown, a direct current field, produced by means of a direct current source 44 of volts at l0 milliamperes, is connected in series with a one-henry choke 4S, which prevents the alternating current from an alternating current source 46 from being applied across the direct current source 44. The alternating current source 46, which produces a deviating voltage of approximately 1 volt, is fed by way of a one-microfarad blocking condenser 47 to one terminal 48 of field coil 42, and the other side of the source or generator 46 is connected to the oppositely-disposed terminal 49, to provide an alternating deviation voltage at the frequency at which it is desirable to modulate the microwave source 18. The capacitor 47 prevents the direct current source 44 from being applied across the alternating current source 46. In this manner, direct current and the alternating current fields may be applied simultaneously to the field coil 42 in order to rotate the polarized energy in the rotator and to amplitude-modulate or frequency-modulate the microwave source 18 in a manner which will be presently described in detail.

The input T section 16 is connected to the ferrite rotator 14 by means of a flange S1 in register with the ferrite rotator input flange 34a, and includes a square section of waveguide 53, which is shown in Fig. l, rotated by an angle of with respect to the output section of square waveguide 26. Extending from the input T section, and integral therewith, is the rectangular arm 54, which is terminated in a well-known matched termination 55, secured by a flange 56 integral with the rectangular arm 54 and connected to the termination 55 by means of screws 58 and termination flange 57. rll'he termination 57 consists of a section of waveguide filled with Polyiron 60 or any similar non-reflecting material adapted to Lit) absorb electrical energy. The other arm of the T section 16 is connected directly to the source of microwave energy 18 which feeds the square waveguide section 59.

In the present embodiment the microwave generator 18 can consist of a magnetron or klystron oscillator. However any type of microwave generator, and any standard-type matching means for transferring energy from a square, circular, or rectangular waveguide into the ferrite rotator 14 may be used, such as a circular matching button, a quarter wave transformer, irises, and the like provided that the proper polarization and mode of energy is thereby observed.

In operation, therefore, microwave energy entering the square waveguide 53 will enter the ferrite rotator 14, where it is rotated a predetermined number of degrees, preferably 45, by the fixed magnetic field applied to the ferrite element 36. `The energy thus rotated enters the square waveguide portion 26 of the output T section 12 and is propagated by means of antenna or load 22. AS noted, energy reflected from the load 22 will be isolated from the microwave source 18 and become rotated another 45 and appear in the matched termination 55. In like manner, microwave energy entering rectangular arm 27 as a result of a small deviation from the 45 rotation acquired in rotator 14 in response to the applied alternating current field set up by alternating current source 46 can be directed toward the microwave source 18. The character of this energy is made reactive or resistive at the microwave source by adjusting slidable element 32. As is known, many microwave sources operating into a varying reactive load are frequency modulated. Therefore, by adjusting the position of shorting bar 32 by rotating adjustable screw 31, which is threadably inserted through tapped section of waveguide 30, the short circuit element can be adjusted to produce a reactive or resistive load on the microwave source according to the distance in eighthor quarter-wavelengths through which the plunger 32 is moved from a zero point, usually occurring in a short distance outside the outer edge of the main waveguide. For example, when the plunger 32 is set at a distance of oneeighth wavelength, the load reflected is purely resistive and produces amplitude modulation. Setting the plunger at a distance of a quarter wavelength, the load becomes purely reactive and produces frequency modulation. The rate at which the microwave source is frequency modulated depends upon the frequency of the applied alternating current producing the deviation from the 45 angle of fixed rotation. In this manner, the characteristic of the load presented to the microwave source becomes purely resistive every quarter wavelength setting of the plunger 32 and the effective coupling or resistive load is then varied by the alternating current field to produce amplitude modulation by switching in and out this variable resistive load. It should be further understood that. at distances greater than a one-eighth wavelength and less than one quarter wavelength along the arm, a mixture of frequency and amplitude modulation is produced. ln other words, the phase of the modulation, as seen by the micro wave source, will be solely a function of the plunger position which determines the line length between thc microwave source and the plunger arm 32. In the present instance, a deviation of one degree from the 45 center rotation can be produced by approximately one volt alternating current applied to the field coil 42 to produce a modulating voltage which is applied to the microwave source to pull the frequency of the microwave generator in a desired manner. By means of the small variation from 45, only a small amount of modulating energy will be translated and lost in the matched termination 55. If the displacement from the 45 zero position is achieved at a relatively rapid rate, the microwave energy reflected into the microwave source will be mod` ulated at the modulating frequency.

Referring now to Fig. 2, the modulator impedance is shown superimposed on a Rieke diagram adjusted for frequency modulation. As the ferrite rotator achieves polarization rotation back and forth around the mismatch or deviating voltage plotted as modulator impedance is shown moving perpendicular to the contours of frequency, thereby frequency modulating the microwave source by means of its pulling characteristics. A typical Rieke diagram is shown on page 4l of vol. 6 of the Radiation Laboratories Series entitled, "Microwave Magnetrons" by George D. Collins. Thus, a one-volt alternating current at approximately a hundred cycles will produce a peak deviation of approximately tive kilocycles in one direction from the center X-band frequency.

Fig. 3 shows the ferrite circulator used as an amplitude modulation device. When the short-circuit bar 32 is set at a one-quarter wavelength position, the impedance change is perpendicular to the contours of constant power, and is shown represented as PD plus or minus l0 or 20 percent of the normal power output Po.

Thus, by rotating the mismatch by changing the position of plunger 32, it is possible to change the impedance perpendicular to the contours of constant power, the center frequency being represented as fn. In general, this change will be parallel to the contours of constant frequency, so that virtually no frequency modulation will be obtained as the amplitude of the microwave source is varied by the changing resistive load or mismatch. As the modulation is displaced from the center or rest position of 45", the modulator impedance moves out, as noted, parallel to the contours of constant frequency, the plus or minus depending upon the direction in which the ferrite is rotated.

It is to be understood that the above-described arrangements are illustrative of the application of the principles of the invention. Numerous other arrangements may be devised by those skilled in the art without departing from the spirit and scope of the invention. Moreover, the invention is not limited to the particular details of construction, materials and processes described, as many equivalents will suggest themselves to those skilled in the art. It is accordingly desired that the appended claims be given a broad interpretation commensurate with the scope of the invention within the art.

What is claimed is:

l. A waveguide rotating device for providing modulation of a microwave source of electromagnetic energy comprising first and second T sections of waveguide each having a pair of cross-polarized arms, said sections rotated to each other by a predetermined angular distance with respect to one of said polarizations, the first of said sections adapted to be connected to a source of microwave energy, the second of said sections adapted to be connected to a load, a ferrite rotating element interposed between and connecting said sections, a matching load connected to one of said arms of said first T section, an adjustable short circuit in register with one of said arms of said second T section, means for applying a direct current to said rotating element to rotate the plane of polarization of said microwave energy by said predetermined angular distance, means for impressing an alternating current on said direct current for producing a deviation of the polarization of said microwave energy, and means for controlling the direction of propagation of a portion of said deviated microwave energy in said first T section.

2. A waveguide system for providing modulation of a microwave source of energy comprising first and second T sections of waveguide each having a pair of crosspolarized arms, a polarization rotating element adapted to receive and rotate linearly polarized microwave energy from a microwave source interposed between and connecting said T sections, a load connected to an arm of one of said T sections, an arm of the other of said T sections connected a microwave source of energy, means for producing a deviation of the polarization of said microwave energy in said rotating element, means including short circuit means connected to the other arm of said one of said T sections adapted to present a reactive or resistive mismatch to said source of microwave energy, thereby to initiate corresponding frequency or amplitude modulation of said microwave source, and a matched termination connected to an arm of the other said T sections adjacent said microwave source to absorb energy reflected from said load.

3. In combination, a first section of square waveguide adapted to receive and support electromagnetic energy in a plurality of modes, an input port having a rectangular aperture for propagating linearly polarized electromagnetic wave energy in said guide, a rectangular input arm connected at right angles to said waveguide structure and arranged to introduce energy into said square waveguide in a mode cross-polarized with respect to energy entering said input port, a second section of square waveguide rotated approximately 45 with respect to one polarization in said first section of waveguide and a nonreciprocal rotating element for rotating the plane of polarization of said electromagnetic energy interposed between and connecting said first and second sections, an alternating magnetic field applied to said rotating element to provide a deviation from said rotated energy, and a short circuit connected to said input arm to reflect a portion of said deviated energy toward said input port.

4. ln combination, a first T section of waveguide having cross-polarized rectangular input arms and an output arm7 a source of microwave energy feeding one of said input arms, a second T section of waveguide having a pair of cross-polarized rectangular input arms and an output arm, said second section related by an angle of approximately 45 to said one polarization in the output arm of said first section, a nonreciprocal rotating element for rotating the plane of polarization of said energy by said angle interposed between and connecting said sections, said rotating element adapted to provide a xed rotation of approximately 45 and a deviation rotation from said fixed rotation in response to an alternating magnetic eld applied thereto, a matched isolating termination connected to an input arm of said first section other than the input arm to which said rotating element is already connected, a load connected to said output arm of said second section, and an adjustable short-circuit device in register with the other of said input arms of said second section, whereby a portion of said deviated microwave energy is returned to said microwave source.

5. A waveguide rotating device for providing modulation of a microwave source of electromagnetic energy comprising a first section of square waveguide having an input port adapted to be connected to a source of microwave energy and an output port adapted to support electromagnetic energy in two polarizations, a polarization-selective waveguide connected to said output port, an input arm fixed to one side of said waveguide section adapted to couple to and from one of said polarizations in said first section, a second section of waveguide having an input port, an output port adapted to be connected to a load, and an input arm fixed to one side of said waveguide section and adapted to couple to and from one of said polarizations in said second section that is related by an angle to said one polarization in said first section, a nonreciprocal ferrite rotating element for rotating the plane of polarization of said energy by said angle interposed between and connecting said sections, a matched termination connected to said input arm of said first section for absorbing said energy reflected from said load, thereby isolating energy from said microwave source. au adjustable shorting means connected to the input arm of said second section, and a separate source of alternating current connected to said rotating element for providing a deviation rotation from said aforementioned angle to direct a portion of said deviated energy into said adjustable shorting means.

6. A waveguide rotating device for providing modulation of a microwave source of electromagnetic energy comprising a first section of waveguide having an input port adapted to be connected to a source of microwave energy and an output port adapted to support electromagnetic energy in two polarizations, a rectangular input arm fixed to one side of said waveguide section and adapted to couple to and from one of said polarizations in said tirst section, a second section of waveguide having an input port, an output port adapted to be connected to a load, and an input arm fixed to one side of said waveguide section and adapted to couple to and from one of said polarizations in said second section that is related by an angle to Said one polarization in said first section, an antireciprocal rotating element for rotating the plane of polarization of said energy by said angle interposed between and connecting said sections, said rotating element comprising a waveguide receptive of said electromagnetic energy, ferrite element means positioned within said waveguide, a magnetic field-producing means surrounding said waveguide in the region of said ferrite element, separate means for supplying a direct current to said field-producing means, and means for impressing an alternating current on said direct current to produce a deviation of the polarization of said microwave energy, and means in register with said input arm of said second section to direct a portion of said deviated microwave energy back into said microwave source.

7. A waveguide rotating device for providing modulation of a microwave source of electromagnetic energy comprising first and second cross-polarized T sections of waveguide adapted to support electromagnetic energy in two polarizations, said T sections being rotated a predetermined angular distance with respect to one of said polarizations, a ferrite rotating element interposed between and connecting said sections, means for applying a direct current to said rotating element to'rotate the plane of polarization of said microwave energy by said angular distance, means for impressing an alternating current on said direct current for producing a deviation of the polarization of said microwave energy, and means for controlling the direction of propagation of a portion of said deviated energy in said first cross-polarized section.

8. A waveguide rotating device for providing modulation of a microwave source of electromagnetic energy comprising a first section of waveguide having an input port adapted to be connected to a source of microwave energy and an output port adapted to support electromagnetic energy in two polarizations, an input arm fixed to one side of said first waveguide section adapted to couple to and from one of said polarizations in said first section, a second section of waveguide having an input port, an output port adapted to be connected to a load, and an input arm fixed to one side of said second waveguide section and adapted to couple to and from one of said polarizations in said second section that is related by an angle to said one polarization in said first section, an antireciprocal rotating element for rotating the plane of polarization of said energy by said angle interposed between and connecting said sections, a matched termination connected to said input arm of said first section for absorbing energy refiected from said load, an adiustable shorting means, connected to the input arm of said second section, and a separate source of alternating energy connected to said rotating element for providing a deviation rotation from said aforementioned angle to direct a portion of said deviated energy into said shorting means for reflection in the direction of said source of microwave energy.

9. A waveguide rotating device for providing modulation of a microwave source of electromagnetic energy comprising first and second cross-polarized T sections of waveguide rotated in relation to each other by a predetermined angular distance with respect to one of said polarizations, a ferrite rotating element interposed between and connecting said sections, means for applying a direct current to said rotating element to rotate the plane of polarization of said microwave energy by said angular distance, means for impressing an alternating current on said direct current for producing a deviation of the polarization of said microwave energy, and means connected to one of said T sections for controlling the phase of said deviated microwave energy propagating through said T sections.

10. A waveguide rotating device for providing modulation of a microwave source of electromagnetic energy comprising first and second T sections of wavegiude each having a pair of cross-polarized arms, said sections rotated to each other by a predetermined angular distance with respect to one of said polarizations, the first of said sections adapted to be connected to a source of microwave energy, the second of said sections adapted to be connected to a load, a ferrite rotating element interposed between and connecting said sections, a matching load connected to one of said arms of said first T section, means for applying a direct current to said rotating element to rotate the plane of polarization of said microwave energy by said predetermined angular distance, means for impressing an alternating current on said direct current to produce a deviation of the polarization of said microwave energy, and shorting means in register with said second section adapted to control the phase of said deviated microwave energy propagating toward said microwave source.

11. A waveguide rotating device for providing modulation of a microwave source of electromagnetic energy comprising first and second T sections of waveguide each having a pair of cross-polarized arms, said sections rotated to each other by a predetermined angular distance with respect to one of said polarizations, the first of said sections adapted to be connected to a source of microwave energy, the second of said sections adapted to be connected to a load, a ferrite rotating element interposed between and connecting said sections, a matching load connected to one of said arms of said first T section, means for applying a direct current to said rotating element to rotate the plane of polarization of said microwave energy by said predetermined angular distance, means for impressing an alternating current on said direct current to produce a deviation voltage, and adjustable short circuit means connected to one of said arms of said second T section adapted to present a resistive or reactive load to said microwave source depending upon the position of said short-circuit means along said arm of said Second T section, thereby producing amplitude or frequency modulation of said microwave source.

12. A waveguide rotating device for providing modulation of a microwave source of electromagnetic energy comprising rst and second T sections of waveguide each section having a pair of cross-polarized arms, said sections rotated to each other by a predetermined angular distance with respect to one of said polarizations, the first 0f said sections adapted to be connected to a source of microwave energy, the second of said sections adapted to be connected to a load, a ferrite rotating element interposed between and connecting said sections, a matching load connected to one of said arms of said first T section, means for applying a direct current to said rotating element to rotate the plane of polarization of said microwave energy by said predetermined angular distance, means for impressing an alternating current on said direct current to produce a deviation of the polarization of said microwave energy, and waveguide shorting means in register with one of the arms of said second section adapted to control the phase of said deviation voltage propagating toward said microwave source.

13. A system for providing modulation of a microwave source of energy comprising a polarization rotating element adapted to receive and rotate linearly polarized microwave energy from a microwave source, a load connected to said rotating element, means for producing a deviation of the polarization of said microwave energy in said rotating element, and means between said load and said rotating element to direct said deviated microwave energy back into said microwave source to shift the frequency of said microwave source in response to amplitude variations in the reactive characteristics of said deviated voltage.

14. A system for providing modulation of a microwave source of energy comprising a polarization rotating element adapted to receive and rotate linearly polarized microwave energy from a microwave source, a load connected to said rotating element, means for producing a deviation voltage in said rotating element, and means including short-circuit means between said load and said rotating element positioned in a manner to reflect a portion of said deviation voltage through said rotating element back to said microwave source, thereby to present either a reactive or resistive load to said source of microwave energy to produce frequency or amplitude modulation thereof.

15. A system for providing modulation of a microwave source of energy comprising a polarization ferrite rotating element adapted to receive and rotate linearly polarized microwave energy from a microwave source, a load connected to said rotating element, means for producing a deviation of the polarization of said microwave energy in said rotating element, and means including short circuit means connected between said load and said rotating element and positioned in a manner adapted to reilect a portion of said microwave energy through said rotating element back into said source of microwave energy, thereby to present a reactive or resistive mismatch to said source of microwave energy, thereby to initiate corresponding frequency or amplitude modulation of said microwave source.

References Cited in the file of this patent UNITED STATES PATENTS Hogan May 29, 1956 Mumford Nov. 6, 1956 Anderson Oct. 21, 1958 

